In hydraulic systems using oil, different kinds of hydraulic displacement pumps are known. These include, for example, the vane pump and the gear pump. By the above-mentioned displacement element is to be understood the gears in the case of gear pumps, and the combination of the rotor and vanes in the case of the vane pump. In the case of the vane pump a further component in the form of a pivoting ring is required for the displacement process, although the casing of eccentric configuration may assume the function of the pivoting ring.
Until now the known hydraulic pumps used mainly mineral oils of 20 to 60 cSt (2 to 6.times.10.sup.-5 m.sup.2 /s) at 50.degree. C. Provided an adequate viscosity of the hydraulic liquid is available, no problems are encountered in connection with the known hydraulic pumps; this is also true as regards lubrication.
However, in the case of welding machines, furnace closing machines, machines for treating inflammable materials, etc. attempts have also already been made at using non-inflammable hydraulic liquids, because in these machines a defect in the hydraulic system could immediately cause fire throughout the entire plant. Such non-inflammable hydraulic liquids differ significantly from the customary mineral hydraulic oils. For example, the lubricating properties are particularly critical in the case of oil-in-water emulsions, which are of very low viscosity on account of their low oil content of 2 to 5% and which are more similar to water from the point of view of lubricating effect as well as that of compressibility and protection against corrosion. Water-based operating media (95% water) have already been found for mining applications, and the design of presses, etc. and plants have been made to enable such media to be used. In the case of low-viscosity pumped media, however, special designs have had to be developed for the various system components, such as for example pumps and valves, which have the disadvantages of high cost and a short life.
There continues to be a demand for hydraulic systems for pumped media in the form of emulsions, because these may have the advantage that one and the same medium can be used for lubrication and cooling. In the case of machine tools, for example, it is presently known to use drilling oil or other emulsions for cooling, but mineral oil of higher viscosity for lubrication. Here there is the danger of mixing and hence pollution of one medium for the other.
Apart from the demand for a hydraulic liquid which is difficult to ignite and which is suitable both for lubrication and cooling, so that only a single liquid is used, it is also desirable to conform to the further condition of good heat dissipation or thermal conductivity. As is known, the thermal conductivity of mineral oils is lower than that of emulsions.
Hence, while the desire for using lowviscosity pumping media in place of mineral oils exists, nevertheless there have not until now been provided hydraulic pumps which can be manufactured and operated economically and which have a long life. In the use of gear- and also vane pumps it is found when processing and/or conveying low-viscosity hydraulic fluids, that the thickness of the lubricating film becomes so small that the parts which move relative to each other come into a mixed friction region and are thus subjected to considerable wear.
This problem is particularly prevalent in the case of the gear pump, the so-called "goggle pump", in which the gears are disposed in bearing plates ("goggles"). The latter is subjected to the pumping pressure and acts on the end face side of the gears, so that a pressure-dependent sealing action is produced. When using low-viscosity pumping media the sealing gap must be reduced for the avoidance of leakages and for the achievement of a high efficiency which, however, increases the danger of bringing the moving parts into the mixed friction region. Hence the provision of an economical hydraulic pump for emulsions has until now not yet been achieved.